Electronics notes/Diodes

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⚠ This is for beginners and very much by a beginner / hobbyist

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See also Category:Electronics.



This article/section is a stub — some half-sorted notes, not necessarily checked, not necessarily correct. Feel free to ignore, or tell me about it.

Diodes were developed in vacuum tube days, around the same time semiconductor diodes were. We now mean semiconductor diodes, unless noted otherwise.

The basic introduction to semiconductor diodes is that they conduct one way, and not the other. (a little more precisely, have very high resistance in one direction and very low resistance in the other).

That's may be enough knowledge for many applications around power (supplies), but around signals and in general (and power in specifics), there are some subtleties to their behaviour.

For starters, look at the diode I-V curve.

A diode is largely characterized by:

  • Vf forward voltage drop
  • Vbr reverse breakdown voltage
  • current limit it will manage

Common wishes for diodes

  • Large current - for power control) (e.g. 1N4000, 1N5400 series)
  • low voltage drop - e.g. when doing polarity protection (e.g. ~0.7 for silicon, 0.3 for schottky, 0.2V for germanium)
  • fast switching - for signal use. Often implies lower current.
  • max reverse bias (commonly in the range of a few dozen to a few hundred)
  • reverse recovery (e.g. schottky)
  • low leakage (all diodes leak on the order of a few microamps in reverse)
  • high breakdown voltage

In forward bias

Diodes don't conduct current until forward bias (voltage) is higher than the diode's forward voltage drop, Vf.

This is usually in the 0.2 .. 0.7V range (and correlated to specific types, e.g. ~0.7 for silicon, 0.3 for schottky, 0.2V for germanium), but can be engineered higher (and will also be higher for diodes in series).

Above that voltage it they are pretty good conductor. It's still a semiconductor, though, so presents a small resistance. You can model a real diode as an ideal diode (the basic description of 'conducts one way and not the other') in series with a resistor.

That resistance actually varies with current (see the fact that the forward conduction part of the diode I-V curve so is not quite a straight line), so technically nonlinear, but in most uses this doesn't matter.

That voltage is also the voltage they drop

functionally this is usually is a ignoreable detail, sometimes in your way, and sometimes even useful.
this voltage is almost independent of current
that voltage times the current passed is wasted as heat
For example, running 3A through a 0.6V-drop diode means ~1.8watt wasted, as heat.

In reverse bias:

No current flows (aside from a tiny leak current).

...until the reverse breakdown voltage, and then they start conducting anyway. The behaviour is called Avalance breakdown, naming relevant semiconductor behaviour[1].

That voltage is often 50V or more, and can be engineered to be higher.

Which means you can ignore this effect in in a lot of low-voltage electronics.

Most diodes are not made for this, will be easily damaged. (Again, look at the I-V curve: the reverse current curve is much steeper, meaning there is little difference between 'conducts a little' and 'burns itself up unless something else limits current')

Zener diodes are roughly the only type that are made for this. Their reverse breakdown voltage is intentionally lower (see Zener applications), and the I-V curve is often a little less steep, meaning there is more voltage range they are comfortable with, and is easier for you to intentionally use. See the section on Zeners below.

LEDs are somewhat unusual diodes.

They have higher Vf, most between 1.4V and 4.5V,
their design focuses on light, means they can deal with very little current,
they have negative resistance curve (putting current through them heats them up, lowering the resistance and passing more current - thermal runaway until it burns)

The are also more fragile in reverse.

You don't want to use LEDs as diodes for power, but they have some uses around signals, where you know current is never high.

See also: http://www.stephenpeek.co.uk/Electronics/diode/diode.htm

Zener diode (type/behaviour)

This article/section is a stub — some half-sorted notes, not necessarily checked, not necessarily correct. Feel free to ignore, or tell me about it.
💤 For context:

Reverse breakdown refers to when you put voltage on a diode the direction it's supposed to block in, and have that be high enough that it starts conducting in reverse.

Is this exactly what you want diodes to not do? Yup.

All diodes do this at some point, at what we refer to as its zener voltage.

  • at this point, they start conducting in reverse
  • at some point above this, they will be damaged.

On regular diodes,

  • This is easy to avoid, because regular diodes have a zener voltage on the order of dozens of volts, and most circuits try to stay low voltage
  • the zener voltage and the point at which they become damaged are close together (not well controlled, or well specced, so even if you want to use the effect, you will have a hard time)

A Zener diode, in forward bias, acts like a completely regular diode (and with similar voltage drop, usually roughly 0.6V .. 0.7V drop, sometimes lower(verify))

Their difference is in reverse behaviour:

  • Their Zener voltage is engineered to be at a voltage lower than in regular diodes - low enough to be used in typical low-voltage electronics (often between 2.4V and 7V, apparently up to 33V. Lower-than-2.4V Zeners are actually imitations of Zener behaviour, and their behaviour is a little different(verify))
  • they have a wider reverse breakdown region
  • they are better-characterized in that region
  • they are more robust within that region

The better-defined reverse breakdown region make it exploitable in its own right, e.g.

  • voltage clamping protection (and other purposes where they effectively act as a sort of relief valve)
e.g. say you have a 5V-powered device, and added a 5.1V zener. A transient spike in the input will be suppressed to a good amount (and because it's transient will not cause temperature issues in the zener)
  • simple, shunt-style regulation
(TODO: simple zener regulation image)
not efficient, not the most stable, so for more than a little current you want other regulation
but e.g. great to protect signal inputs
  • fixed voltage references
  • clipping signals
e.g. one for DC, two for an AC waveform
in audio for distortion (see fuzz boxes)
avoiding op amp phase inversion

The current and heat dissipation varies between zeners, often range of milliwatts for voltage reference, a few hundred milliwatts for simple regulation, up to a Watt or so.

Since you rarely want to move a lot of current in reverse bias, circuits will often see some sort of current limitation (a resistor or such).

See also:

Varactor diode (type/behaviour)

This article/section is a stub — some half-sorted notes, not necessarily checked, not necessarily correct. Feel free to ignore, or tell me about it.

Varactor diodes, a.k.a. varicap diodes, variable capacitance diodes, variable reactance diodes or tuning diode

...are aimed exploiting the voltage-dependent capacitance of a reverse-biased p–n junction. All diodes do this, but for many others it is small and does not vary much; varactor diodes are manufactured for this property to be more exploitable.


Avalanche diode (spec/type)

Avalance diodes usually refer to diodes that are made to imitate Zener behaviour, yet will work for higher voltages (100 ~ 300V) than true Zeners can.

Avalance diodes may be found categorized (or referred to) as Zener diodes.

See also:

Rectifier diode (spec)

Rectification is a circuit typically using diodes, not a type of diode.

However, you would prefer specific specs for diodes used in rectification, and you find such diodes in their own section when you're looking to buy them.

Also, since you usually combine two or four of them, there are also single components that contain two or four suitable diodes, reducing component count and possibly simplifying heat sinking.

Types and materials

Silicon diode

Common, cheap.

Forward voltage drop of about 0.7V.

Germanium diode

Forward voltage drop of about 0.3V.

Schottky diode (type)

This article/section is a stub — some half-sorted notes, not necessarily checked, not necessarily correct. Feel free to ignore, or tell me about it.

Schottky diode, a.k.a. barrier diode, refers to a metal-semiconductor construction.

Compared to most other diodes (which tend to be semiconductor-semiconductor), Schottky diodes have

a lower forward voltage drop, on the order of 0.15V .. 0.45V rather than the more typical 0.6V .. 0.7V.
switch faster than some.
faster reverse recovery time (due to lower parasitic capacitance?)


lower reverse voltage ratings
higher reverse leakage current

That lower drop is useful

  • in reverse-polarity protection (less power is wasted when it's inline all the time on something capable of moderate current), useful e.g. on DC sockets.
  • around solar panels, e.g. bypassing broken/shadowed panels and/or protecting battery discharge through the panels(verify)
  • voltage clamping is more efficient
  • in switch-mode power supplies, for similar efficiency reasons (the fast recovery is also useful)
  • certain fast switching
  • some higher frequency filtering (potentially up to GHzes, where regular diodes tend to MHzes)

However, the lower reverse voltage and higher reverse leakage mean a schottky requires more design considerations (e.g. thermal runaway is more likely), and due to this relative fragility, using it as a generic generic diode is probably a bad idea.

See also:

Current regulating diode (type)

Signal diode, switching diode (spec)

Fast recovery diode (spec)


Bypass diode / flyback protection diode (application)

A bypass diode is a diode used to protect something against reverse biasing - against voltage going the wrong way.

Also known as flyback diode, kickback diode, catch diode, snubber diode, freewheeling diode, suppressor diode, commutating diode, antiparallel diode, and more. (Some names refer more specifically to more transient reverse biasing, or the causes behind that)


  • inductors in general can store energy (in the magnetic field)
some because sudden changes (e.g. solenoid disconnecting)
and larger things that move (solenoids, motors) a little more so (movement that translates back to magnetic field)
They can often easily deliver a few times the voltage they usually operate at (varying with case). This can seriously stress the circuit components directly around it, depending on design.
The use of a diode in parallel, reverse-biased to how you drive it, is one easy way to bleed this short-lived current
yes, you're essentially making a one-way short-circuit, which is why it also affects the braking behaviour of coil-style actuators: the collapse of the magnetic field is usually slowed(verify)
on the spike shape:
  • In a series of solar cells, shading one cell among multiple leads that one cell it to be reverse biased relative to the others, meaning that panel gets fed current from the others, which would heat it and may eventually damage it.
A bypass diode in parallel with each cel llets the current-in-the-wrong-direction pass alongside the cell instead.


  • If the load is bidirectional (e.g. bidirectional motor driver) then the flyback can happen in either direction, and you can't protect it in the way described above
For example, in the case of a H-bridge driver, you need four diodes for protection in both directions
  • Protection diodes should be able to take a decent load - may well be a few amps for a short while (depends a lot on the load)
bog-standard diode like 1N4001-1N4007 series can be used - for lower frequencies
for higher frequencies you need to look around more, probably for schottky diodes
  • Protection diodes should have a voltage rating of at least the voltage you are applying
whenever significant energy can get stored (e.g. in case of coils), leeway is a good idea
  • Protection diodes should be robust to reverse biasing (themselves)
  • You may have preference for a fast-acting diode (e.g. Schottky) to minimize the time the reverse bias is applied (though note they have lower reverse-bias ratings)
  • Sometimes you may also care about low voltage drop (which can matter e.g. in solar and battery applications, minimizing loss).
  • If reaction time is not so important (or low cost is considered more important), cheaper general-purpose diodes such as the 1N400x series can be used.


See also:

Blocking diode (application)

Clamp diode, protection diode, diode clipping (application)

Protection diodes in ICs

Clamper circuit, diode clamp (application)

Rectifier bridge (simple design)


(for LEDin lighting, see Lightbulb_notes#LED)

LEDs convert a higher percentage of energy going through them to light than, say, classic lightbulbs (where a lot of the energy becomes heat, i.e. IR radiation).

That sounds awesomely efficient, though there's a footnote to that: getting from AC mains voltage to few-volt DC voltage. That conversion is not very efficient to do, particularly not for the quite-low currents involved when each just powers a few LEDs.

LEDs do act as diodes ('Light Emitting Diodes', after all), in that they conduct/work only in one direction. Their ratings as diodes is pretty poor, though. They are slower, and their reverse breakdown is close, so in many situations you'ld want to add a serious diode.

LED of different types usually need something like 2V or 3V forward voltage before they start emitting light. (The extremes, including some less usual types, seem to be 1.6V and approx. 4.4V)(verify).


LEDs have an I-V curve ("how much current will flow when I vary voltage?") that is not only nonlinear, but also very steep exactly near the point you would like to operate them at, particularly if you want to drive them at decent brightness and have them live long.

they have a threshold below which they do nothing
a fairly flat bit of still low current (and they will be relatively dim here)
quick-and-dirty uses operate them here, e.g. by using a somewhat large resistor.
and then shoots up, from little current to too much, over a volt or two

The exact curve varies with

  • LED type (that search shows different curves for the different classic LED colors),
  • environment temperature - itself largely determined by the current currently going through it
  • production line variation

Indicator LEDs tend to be specced for a continuous current of ~10mA. Superbright for perhaps ~20mA, and low-current LEDs perhaps 2mA.

(figures for classical LEDs. SMDs seem to have more range)

Keep in mind that

most LEDs already give off some light at 1mA.
these are continuous-use maximums.
you can often pulse higher currents on a low duty cycle. Some LEDs give move explicit spec sheets - usually the ones that considered heat sinking enough to actually put some numbers to this

All that makes it very hard to create a LED driver that is entirely universal and plug and play.

For individual LEDs at small current, the current-limiting resistor is typical approach, because it which works fine on things like indicator LEDs where we care more about lifetime and basic visibility than maximum brightness or maximum efficiency. And currents are low.

When driving powerful LEDs, though, we do care (also because of waste heat), and will generally use a current-limiting LED driver, for which there are now plenty of ICs, and supplies centered around them.

LED response time
Current limiting resistor

A LED by itself has negligible resistance.

So it will easily pass much more current than it can stand, and you need something to limit current.

The simplest current limiting is a resistor in series - a current limiting resistor.

For most combinations of LEDs, PCB voltages, and LED current limits, this works out as a resistor on the order of 100 ohm to 1kOhm.

The main upside is that it's very simple.

The downside is that since the resistor passes the same current as the LED, it will also waste some power, putting that into heat instead of light.

For just a few indicator LEDs, it's order of 10mW for each, which is so ittle that it's rarely worth it energy-wise or component-wise to drive them more efficiently.

For room lighting with LED, it starts being a significant amount of energy, and some form of LED driver becomes clearly better.


Being a diode, the LED drops a fixed voltage, its Vf. For specific-colored LEDs, that's usually somewhere between 1.6 and 4.5 volts.

Your calculations aim to have the resistor drop just enough of the (leftover) voltage that the current through it and something in series with it is at/below some target current.

...in this case the specific LED's rated maximum continuous current (preferably a little below it so it lives longer).

Often your givens are the supply voltage, the LED's voltage drop Vf, and the LED's maximum continuous current rating you want to stay under.

You'll usually use:

     Vin - VLED
R = -----------

Chances are you'll grab the next-highest standard-valued resistor. If you want to know the current for it, the following reformulation is useful:

ILED = (Vin - VLED) / R

For example, to drive a ~2.2V red LED, aiming to stay under 20mA, and using standard-value (e.g. cheap carbon 5%) resistors, you might choose:

  • on 3.3V: 100Ω for 11mA, maybe 220Ω for 5mA
  • on 5V: 220Ω for 12mA, maybe 330Ω for 8mA or 150Ω for 19mA,
  • on 9V: 470Ω for 14mA
  • on 12V: 560Ω for 17mA, or 1kΩ for 10mA, or maybe 470Ω for 21mA.

Side note: A LED's max-current rating is for a good part limited by the ability to sink the generated heat, and assumes continuous current. When you're only ever pulsing the LED with a lowish duty cycle, you can get away with higher currents. For example, TV remote IR leds can get away with being driven brighter in part because their pulsed protocols are almost always low-duty-cycle. And you might do it for visible LEDs because when using something like PWM, for part of the curve current drops off faster than the human-apparent intensity.

LED drivers

A resistor may be simple enough for a single LED, and the losses too small to consider, but above perhaps a few hundred mA, it becomes worth it energy-wise to invest in components to drive the LED more efficiently.

This also means we can more safely aim for a current that is nearer its maximum brightness with less risk of burning the LED, because with better design it's easier to sit on a specific part of its nonlinear IV curve.

There are two basic approaches:

Constant-current LED driver
Constant-voltage LED driver
Multiple LEDs
Common cathode versus common anode
Series versus parallel


LEDs can be dimmed, in two basic ways:

  • Control the current
The amount of light isn't linear, but when humans adjust a knob, they tend to be practical about it, and if digital you can compensate.
The simple analog circuit requires little more than a transistor (being a current amplifier) and a voltage divider with a potmeter to control it, plus some fine tuning for the transistor's gain
...plus some thought on how to never drive it with too much current

  • Blink it very fast, typically using PWM, or possibly a relatively analog oscillating circuit.
Once the cycle in which it blinks is 100Hz or faster, we humans only really perceive the average amount of light that comes out. When you have a microcontroller this tends to be moderately easy. There are some cheaper ICs that can do much the same.
note that the current drops off faster than the intensity apparent to humans, meaning you can save a little power
...and/or get a little more lifetime out of a LED, as it's heat (more than anything else) that destroys LEDs

It also means that on lower duty cycles you can drive them a slightly higher currents than you would use for continuous driving. But since LEDs typically heat up faster than they cool down, this is not "half the duty cycle means twice the power" stuff. Datasheets may spec some of these details, and there are some rules of thumb you can follow if they don't.

Also, it's not really worth it to do this for apparent brightness, because you need to lower the duty cycle.

Still, it's useful for things like an infrared TV remotes. Because the protocols used are quite low duty cycle (typically simple bit-trains, where highs are themselves a 38kHz blockwave rather than continuous, and with relatively large intervals between repeated commands, so in this use are guaranteed to have a pretty low duty cycle), you may be able to drive them at a few times their rated continuous current, which means they carry further.


LED designed made for room lighting are designed to deal with hundreds of mA or more. Often because they're a large area or LEDS side by side (and often in part in series, allowing (or forcing) us to use higher voltages).


Silicon-controlled rectifiers (SCR)




See also


LED pixel strings

This article/section is a stub — some half-sorted notes, not necessarily checked, not necessarily correct. Feel free to ignore, or tell me about it.

Each is a LED with built in driver chip (often 3 channels, three LEDS for RGB, roughly 15 to 20mA per-channel, 50 to 60mA if bright white), in something like a 5050 package.

...and that many times in a row.

Adafruit seems to have called the idea of these strips neopixels[2] and that name has caught on somewhat. It does not refer to a special variant.

Many strings are built on a variant that have the same package contain and drive the LEDs. There are also variants where the driver is separate (and usually PWM a handful of LEDs each). They work and control much the same, though are not as handy to put in a LED strip - but can be useful in some other situations.

These packages are not individually addressable as such. Instead, each chips holds one color. When it gets told a new color, it takes the one it was displaying and puts it on its output.

So sending one color shifts everything in the string into the next pixel.

And if you want to change one pixel within a string, you don't say "pixel 5 goes to red now", you send what all colors in the string should now be.

That means you will need to have remembered what's currently on there, change what you want, then replace the entire string's worth of values again.

That said, the entire entire chain updates fast enough you will barely notice it's actually shifting, at least not until you have more than one or two hundred such pixels in a string.

(If you want to think big, say, make a hundreds-of-pixels display, you'll run into bandwidth and latency details, and you'll probably want to split it into multiple strings, controlled individually. It's more work, but not necessarily much pricier.)

Note that a bunch of these LED ICs have relatively strict timing requirements, in which case the speed of replacing the entire string is relatively fixed.

For an idea of speed, the WS2812, at 800kHz, means 1.25µs per bit, *3colors*8bits = 30µs per pixel, so e.g. 100 pixels takes ~3ms. (This is the order of magnitude you can update with it still looking pretty smooth to us)

There are now one or two dozen variations of ICs doing very similar things to each other.

Most of them (noted if it differs)

  • Have 256 levels(verify) (8-bit) per channel (some try to gamma-correct, which does rather matter to color mixing)
  • control 3 channels (for R,G,B) (sometimes 4, then typically RGBW)
  • current draw: specced to draw around 18mA per fully bright color (varies somewhat per type)
so around ~50mA max per single pixel
...for mixed near-whites; non-washed-out colorswill typically draw something like 20 to 30mA.
So e.g. 50 pixels would be ~2.5A max but typically more like 1A to 1.7A
note that there are variants -- which may be labeled exactly the same -- that draw maybe 60% of that (and are that much less bright)

  • You can often use a slightly lower voltage than the rating
but they'll be dimmer
the point at which they will show incorrect colors (because the different colors have different minimum voltages) happens sooner
and below that fail to light at all.
  • 3.3V-level logic won't work with things expecting 5V CMOS logic, but there are several tricks available (see below)

Somewhat common ICs include:

  • WS2801 - SOP-14 or DIP-14 (separate driver; controls a few LEDs)
    • 3-5V (in theory? is this really a 5V device?)
    • shift up to 25MHz (verify)
    • chains with 4 wires (Vcc, Gnd, Data, Clock)
    • More-channel-per-chip variants include WS2803 (SOP-28, DIP-28) for 18 channels (up to 6 RGB LEDs)
    • WS2801 datasheet

  • WS2811 - PLCC6 (integrated in LED), or separate DIP-8 or SOP-8 chip
    • Vcc is ~6V, VLED up to 12V(verify)
    • chains with 3 wires (Vcc, Gnd, Data) - uses specific timing instead of a clock (...but this puts more constraints on the controller)
    • WS2811 datasheet

  • WS2812, WS2812B - PLCC6 (integrated in LED)
    • 6-7V
    • slight improvement over WS2811 based LEDs, identical to control (verify)
    • chains with 3 wires (Vcc, Gnd, Data) - uses specific timing instead of a clock (...but this puts more constraints on the controller)
    • WS2812 datasheet

  • WS2812c seems to be a smaller (2020), lower-brightness, lower-current (5mA per channel), useful for portable things, otherwise compatible with WS2812B(verify)
  • WS2813 (integrated in LED)
few times higher update speed compared to 2812b(verify)
  • WS2815 (integrated in LED)
12V (which means the voltage drop should matter a little less, so longer runs)
chains with 4 wires, one of which is a backup of the first data line. IO theory, one IC failing means the rest will work, in reality YMMV

  • LPD6803 - SOP-16 / QFN-16 (separate driver)
    • 5-7V
    • shift up to 15MHz (verify)
    • LDP6803 datasheet
    • chains with 4 wires (Vcc, Gnd, Data, Clock)
    • variants include LPD8806, LPD8809

  • LPD8803 - SOP-16, others?(verify) (separate driver)
    • Vcc is 2.7-5.5V, VLED is 3..12V
    • shift up to 20MHz (verify)
    • chains with 4 wires (Vcc, Gnd, Data, Clock)
    • variants include LPD8806, LPD8809

  • HL1606 - SOP-16, others? (separate driver)
    • 3-5V
    • SPI - needs 4 data wires (2 more than most of the others), 6 in total
    • Some sources note it doesn't do PWM, others say it does. I've not looked up what's going on there

I've also seen mention of:

  • More-channel variants of LDPs, e.g. LDP6806 and LDP8806 (6-channel), apparently LDP8809, and probably more.
  • SK6812 (RGBW, separate white; also seen mentioned as RGB, possibly both variants exist?)
  • TM1803 (SOP8, separate driver)
    • 5V? (string can be higher voltage, if you add current limiting resistors for the IC)
  • TM1804 (SOP8)
    • somewhat wider tolerances than TM1803?
  • TM1809 (SOP14)
    • like TM1803, but with 9 output channels, so 3 RGBs
  • SM16716 (SOP16)
  • LPD1109
  • TLS3001 (SOP8/10/14 )
    • 12 bits per channel
  • UCS1904
  • UCS29043
    • RGB
  • UCS2904 (SOP8)
    • RGBW
  • GS8208
    • 12V
  • APA102
    • 5V
    • SPI - updates faster than WS2812(verify), and chains with 4 wires
    • less flickering with low lights (because Faster PWM)


  • On voltage:
    • Some chips can work with and put out at most ~5V (and may start working around 3 point something V).
    • Some will fry if you give them more than ~6V
    • Some can drive up to 12V and are fine with 5-12V.
    • Some are intentionally 12V. 12V means less effect from voltage drop over longer strips(verify)

3.3V tricks

Most of these LEDs are 5V CMOS devices, meaning they are HIGH above 70%*Vcc = ~3.5V, and just won't work when controlled from 3.3V.

See Level shifting for your basic options.

I'd go for the two-diode level shifter, since this is just one line
(note that since ws2812 communicate at 800Kbps, not all optoisolators are fast enough)

There's also a trick that runs one pixel at a lower Vcc (4.3V, via one diode) so that its input is high above >3V, and it outputs 4.3V logic (its Vcc) which is fine for the rest.

The one pixel will be a little less bright, so you either don't use it, or maybe use it for something separate like an indicator.


zero cross circuit